The present invention relates generally to electrodes and more particularly to a new and novel electrode assembly for use in quantifying the levels of various metals in an aqueous solution.
One of the most sensitive techniques presently known for measuring the concentration of various heavy metals in water is anodic stripping voltammetry (ASV). ASV typically comprises a two step process wherein (1) a negative potential is applied at a small film or drop of mercury so that the metal ions in solution are electrochemically reduced and concentrated into the mercury; and (2) after a period of time, the applied potential is slowly scanned in the positive direction, resulting in a peak current at the oxidation potential of each metal proportional to its concentration.
The critical component of this technique is the mercury drop or film electrode. There have been two types of electrodes used with ASV, the hanging-mercury-drop (HMDE) and the mercury-film (MFE). The HMDE is typically a less than 1 mm in diameter drop of mercury suspended from a glass capillary. The problems associated with using the HMDE include the undesirability of working with bulk mercury, its relative "massiveness" with respect to internal diffusion leading to low resolution or prolonged analysis times, and the uncontrolled solution hydrodynamics and diffusion at its surface during the deposition step.
The MFE consists of a flat inlaid disk substrate on which a thin (&lt;100 micron) film of mercury is electrodeposited. Two types of MFE substrates are commonly used, both with inherent problems. The first type of substrate consists of inert materials, such as glassy carbon, graphite or carbide, upon all of which a "mercury droplet film" forms. Even though these types of electrodes have been successfully used for many years in quantitative work, their characteristics are far from ideal. The second type of substrate consists of metals, such as platinum, gold or silver, all of which have a tendency to dissolve into the mercury and to form intermetallic compounds with the metals being analyzed or to eventually convert the film into an amalgam-film, severely limiting the utility of such electrodes.
Although similar to the other noble metals, iridium is substantially harder, more inert, and more expensive. Because of these properties, iridium has generally been overlooked as an electrode substrate. About seven years ago, Kounaves et al. (J. Electrochem. Soc., Vol. 133, pp. 2495-2498 (1986); and J. Electroanal. Chem., Vol. 216, pp. 53-69 (1987)) showed that iridium possesses two properties which make it ideal as a mercury electrode substrate: (1) its solubility in mercury is below 10.sup.-6 percent by weight; and (2) mercury can be electroplated onto an iridium disk to give a uniform film or hemispherical coverage.
Ultramicroelectrodes are electrodes which have a dimension of less than 20 microns in size. They have been shown to possess several unique characteristics in terms of mass transport rates, capacitive charging RC, and reduction in IR drop. They have rapidly become invaluable in a wide range of applications. As with larger electrodes, the most common materials used in their preparation have been platinum, gold or carbon fiber. Platinum ultramicroelectrodes have been fabricated down to diameters as small as 0.003 micron and in various geometries, such as disk, cylindrical and conical. Typical carbon fiber electrodes have diameters in the range of 5-20 microns and are similarly used either as fibers of 1-5 mm length or as disks.
Attempts at fabricating mercury ultramicroelectrodes both on solid substrates and in bulk form have been made by several groups; however, many of the same problems that have plagued the larger electrodes have also limited the utility of the ultramicroelectrodes. The development of iridium-based mercury ultramicroelectrodes was hampered by the lack of commercial available iridium wire of sufficiently small diameter (caused by difficulties in drawing iridium to &lt;127 microns) and the ineffectiveness of the typical etching solutions normally used for platinum or gold.
In J. Electroanal. Chem, Vol. 301, pp. 77-85 (1991), which is incorporated herein by reference, Kounaves et al. describe how they overcame the etching problems with iridium and were successful in developing iridium based mercury ultramicroelectrodes with diameters of 1 to 10 microns. They demonstrated that a stable mercury hemisphere could be formed on the iridium surface and that, by using ASV, the detection of Cd.sup.+2 at 10.sup.-8 M with an analysis time of less than 1 second was feasible.
One disadvantage associated with single ultramicroelectrodes is that they produce currents in the picoamp or nanoamp range and thus usually require specialized instrumentation for reliable measurements. To overcome this limitation, many types of electrode assemblies comprising multielement ultramicroelectrode array configurations have been developed.
Some of the earliest "electroanayltically based" fabrication and experimental work using the disk microelectrode array concept was reported by Gueshi et al., J. Electroanal. Chem., Vol. 89, pp. 247-260 (1978). To confirm their theory for chronopotentiometry and chronoamperometry at partially covered electrodes (i.e., in effect, an array of microelectrodes), they used a gold electrode covered with a photoresist layer. A photolithographic process and mask for the array pattern was used to give a hexagonal array of exposed "gold microelectrodes" on the electrodes surface. Aoki et al., J. Electroanal. Chem., Vol. 125, pp. 315-320 (1981), used a similar technique to produce, on a large glassy carbon substrate, arrays of 157, 114, 51, and 25 circular microelectrodes with radii of 20, 40, 100 and 200 microns, respectively. Caudill et al., Anal. Chem. Vol. 544, pp. 2532-2535 (1982), constructed an electrode array using 5 rows of 20 carbon fibers sandwiched between glass slides, resulting in 10 micron diameter disks. The electrode was used as a channel-type amperometric flow cell detector. Sleszynski et al., Anal. Chem., Vol. 56, pp. 130-135 (1984), used epoxy filled reticulated vitreous carbon (RVC) to construct a two dimensional random order microelectrode array that yielded nearly steady-state currents. Hepel et al., J. Electrochem. Soc. Vol. 133, pp. 752-757 (1986), made Cr and Au microelectrode arrays with over 1 million active electrodes on a 1 cm.sup.2 area, each of 0.75 micron diameter, using electron beam lithography and polymethylmethacrylate resist. Finally, several groups have used porous materials such as polycarbonate membrane or aluminum oxide films to fabricate either recessed random "microhole" arrays, or, in one case, to electrodeposit platinum into the pores resulting in random ordered 0.1 micron disk microelectrodes.
Band arrays, which are also referred to as line or linear microelectrode arrays, have one of their dimensions in the micro-sized domain, while the other dimension may be several orders of magnitude larger. They have usually been fabricated either by sandwiching a thin metal layer between glass layers and then polishing one end or by photolithography.
Band microelectrodes made of metals such as Pt, Au, or Cr are, of course, not amenable for use as substrates for mercury film formation, since coverage of such a long line would require amalgamation, which is undesirable. Mercury deposition on non-metal band microelectrodes would result in a long column of easily detachable small mercury drops, again totally undesirable.
There are several key points in regards to microelectrode arrays reported in the literature. First, a large number of the "microelectrode arrays" are actually linear "arrays" of interdigitated line/band microelectrodes. Second, one of the greatest drawbacks with all of these microelectrode arrays is that their microelectrode surfaces cannot effectively be renewed chemically or by polishing.
In Proceedings of the 12th International Conference of the Association for the Advancement of Rehabilitation Technology, New Orleans, La., pp. 292-293 (June 1989) and Technology Development for a Chronic Neural Interface, Ph.D. Dissertation, Stanford University, Technical Report No. E073-1 (August 1990), which are incorporated herein by reference, Kovacs et al. disclose an implantable electrode assembly which includes an array of iridium ultramicroelectrodes useful as a direct interface between the human nervous system and external prosthetic devices. The iridium ultramicroelectrode array is formed on a suitable substrate by a photolithographic technique with a specialized lift-off patterning process.
None of the above-described ultramicroelectrode arrays involve mercury plated films/spheres and/or have been used for metal ion determination.